1. Field of the Invention
The present invention relates to electric motors and, more particularly, to electric motors having armature winding patterns or distributions that improve commutation and reduce brush sparking and the generation of electromagnetic interference (EMI) during motor operation.
Typical wound-armature electric motors in common use in power tools and appliances utilize an armature having a plurality of wound coils that are serially connected together to define an armature circuit with the ends of each coil connected to adjacent commutator bars. The winding patterns vary from the relatively elementary lap or wave winding patterns to far more complicated winding distributions.
In operation, the armature rotates through the field with the coils successively undergoing commutation as their connected commutator bars pass through the neutral zone of the motor. Although the electro-dynamics of commutation are generally of a complex nature, the direction of current flow through an armature coil is in a first direction as the coil's commutator bars move toward a brush to effect commutation of the coil winding. As the adjacent commutator bars of an armature coil pass under a brush, the brush will span the bars and establish a "make" contact with effectively and momentarily short circuits the coil undergoing commutation. Shortly thereafter, this shunting or short circuit "breaks" and the current to the coil is reversed. During the time that the coil is shorted, the current through the coil undergoing commutation can vary in a complex manner depending upon the inductive and capacitive reactances of the coil and the coil resistance as well as the current flow through the coil just prior to commutation. The breaking of the contact after commutation results in brush sparking or brush fire, the intensity of which is a function of the electro-dynamics of the motor as described above. The brush sparking causes brush erosion and contributes to commutator wear. The process of commutation, including the existance of the brush sparking, is responsible for some, if not all, the electromagnetic and radio frequency radiation generated by the motor during its operation. This electromagnetic interference is typically conducted through the power cord of the motorized device and transferred to a portion of the power distribution system with the EMI typically interferring with the operation of EMI susceptable devices on the common power circuit, these other devices typically including computers, televisions, radio receivers, and the like.
Various national and trans-national standards have been developed to limit EMI sources with these standards limiting the amplitude of the generated EMI to specific signal levels over a broad frequency range. An example of one such standard is "Limits and Methods of Measurement of Radio Frequency Characteristics of Household Electrical Appliances, Portable Tools, and Similar Electrical Apparatus" (Pub. 14, 1975) published by the Comite International Special Des Perturbations Radio-electriques (C.I.S.P.R.). This document establishes EMI radiation standards for motor-driven devices, including portable power tools, in a frequency range of 0.15 MHZ to 300 MHZ.
Traditionally, EMI generated by motor-driven appliances and portable tools has been suppressed using various types of filter networks that include capacitive and/or inductive reactances which bypass or impede the EMI signal flow. Examples of EMI filters include capacitors placed across the power line terminals to provide a low-reactance shunt path for the undesirable higher frequencies as well as more sophisticated filters that include multiple T-section and PI-section filter networks. While traditional filtering techniques have been effective to suppress EMI, the cost of the suppression devices increases the overall cost of the appliance including the cost of fabrication. In the case of portable power tools, the EMI suppression components may also add weight to the tool and tend to increase the physical dimensions of the tool since the designer must provide a tool housing that accommodates the suppression components.